The present invention relates to supporting grids for secondary alkaline storage battery plates of the pasted active material variety. Such plates have a meshwork grid supporting the active material and providing electrical conduction therethrough. The grid is pasted or filled with active material as by pressing, vacuum impregnation from a slurry, etc.
Alkaline batteries may have either soluble, insoluble, or both soluble and insoluble electrodes forming their electrochemical couple. The nickel-zinc battery, for example, contains both a soluble electrode (i.e., Zn) and an insoluble electrode (i.e., NiOOH). The zinc active material is electrolytically formed from zinc oxide which has previously been pasted onto a conductive supporting grid. The NiOOH active material is electrolytically formed from Ni(OH).sub.2 which has previously been pasted onto a conductive supporting grid.
Soluble electrodes dissolve in the electrolyte on discharge and must be recovered therefrom on recharge. The soluble zinc electrode, for example, discharges according to the following reaction: EQU Zn+2OH.sup.- .fwdarw.Zn(OH).sub.2 +2e
The ZnOH.sub.2 then dissolves in the electrolyte to form zincate ion, [Zn(OH).sub.4.sup.-- ], which saturates or precipitates out of the electrolyte (i.e., KOH). In the fully discharged state then, the grid and any residual unreacted zinc (i.e., about 30%) adhering thereto is suspended in an electrolyte-zincate slurry. In order to keep the dissolved zinc from migrating away from its associated grid and to prevent zinc dendrites from growing into contact with adjacent plates during recharge, the zinc electrode is enveloped in thin layers of microporous, separator material which is permeable to the KOH electrolyte but not to the zincate ion. On recharge, the zinc is recovered from the slurry by being electrolytically deposited back onto the grid and residual zinc according to the following reaction: EQU Zn(OH).sub.4.sup.-- +2e.fwdarw.Zn+4OH.sup.-
Ideally, the Zn will deposit on the zinc grid in a highly porous state for better electrolyte contact and will be distributed substantially uniformly across the face of the electrode to avoid the phenomenon known as "shape change." Shape change results from uneven deposition of the zince on the plate on succeeding charge and discharge cycles such that metal in the center of the plate becomes dense and nonreactive while that at the edges of the plate does not want to replate, but rather wants to migrate in the electrolyte. Shape change commonly occurs primarily in plates having large surface areas and uneven current distributions therethrough. Smaller plates with more even current distributions are not as susceptible to shape change as the larger plates.
It is an object of the present invention to provide a lightweight grid for soluble alkaline battery plates of the pasted type which grid promotes low current density dissolution and replating of the reactant resulting in a porous mass thereof substantially uniformly distributed across the face of the electrode throughout repeated charge-discharge cycles.
Insoluble pasted electrodes, on the other hand, have different problems than soluble ones. The insoluble NiOOH plates, for example, undergo a solid state reaction wherein the reactant changes from one solid form to another in situ, but without going into solution. This reaction is as follows: EQU NiOOH+H.sub.2 O+e.fwdarw.Ni(OH).sub.2 +OH.sup.-
wherein protons from the electrolyte and electrons from the circuit move into the nickel oxide crystal lattice and restructure the nickel compound. This reaction is reversed on recharge. Expansion and contraction of the active material mass accompanies this solid state transition and severely stresses pasted plates having conventional monolayer grids. In this regard, the active material separates from the gridwires and often sheds from the plate and falls to the bottom of the cell. Breakage of the gridwires often occurs. All of these detrimental effects combine in time to reduce the performance of the plate. Moreover, NiOOH and Ni(OH).sub.2 compounds are poor conductors of electricity, and hence, costly and weighty inert conductive diluents (e.g., graphite or nickel powder) are typically added to the active material of pasted plates to effectively extend the conductivity of the grid throughout the active material.
It is another object of the present invention to provide a strong, lightweight grid for insoluble alkaline battery plates of the pasted type which grid will become intimately integrated with the active material to promote better retention of the active material, improved contact with the active material and improved electrical conductivity throughout the plate even at reduced conductive diluent loadings.
It is a still further object of the present invention to provide a method of making a laminated, metal-coated thermoplastic battery grid which is divided into a plurality of active-material-retention pockets bounded by conductive buses and including a plurality of grid wires extending throughout the pockets.
These and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.